The invention relates to a cordless telephone comprising at least a base station with a clock, a handset and clock correction means.
The invention also relates to a base station and a handset intended to be used in such a telephone.
The invention also relates to a method of managing clocks for a cordless telephone.
The invention finds important applications, notably for cordless telephones of which the base station has a function of answering machine with date-time stamping of the received messages. For such applications it is necessary to have a calendar in the base station(s).
Patent abstract of Japan no. A-06 120 881 describes a cordless telephone having a base station and a handset which include each a clock circuit. The clock of the base station is controlled by a clock generation circuit in the telephone switch that connects the base station to the telephone line. And clock correction means are provided for correcting the clock of the handset based on the clock of the base station.
It is an object of the invention to propose a cordless telephone having an autonomous, simple and economic time system.
For this purpose, a telephone according to the invention and as described in the opening paragraph is characterized in that said clock correction means comprise:
means for the base station to detect a loss of said clock,
means for at least one handset to generate clock recovery information,
means for the handset to transmit said clock recovery information to the base station.
The invention thus makes use of the fact that handsets have a power supply battery which gives them large autonomy in case of a power cut. When the base station has lost its time, for example, after a power cut, recovery information can be transmitted thereto by the handsets. The invention thus enables to do without the use of a specific autonomous back-up and counting element in the base station to continue calculating the clock notably in case of a power cut.
In a first embodiment of the invention, said base station comprises a non-volatile back-up memory of said clock and said recovery information generating means comprise calculation means for calculating the time elapsed between the detection of the loss of the clock and the transmission of the recovery information.
In a second embodiment of the invention, the handset has a clock slaved to that of the base station for delivering said recovery information.
In this embodiment, the clock of the handsets is normally updated by the base station. This embodiment thus enables to do without the use of a non-volatile memory for regularly backing up the system clock. The use of a non-volatile memory actually has the following drawback: the counting of the duration of a power cut by the handset contains an error that is equal to the delay between the last backing up of the clock in the base station and the start of the power cut. To remedy this, the rate of the clock back-ups (of the order of several seconds) is to be increased. The number of times a non-volatile memory is written is limited, which provides that after a certain period of time (several years) the clock system will no longer function correctly.
Finally, a clock management method according to the invention and as described in the opening paragraph is characterized in that it comprises:
detecting a loss of said clock by the base station,
generating recovery information of said clock in at least one handset,
transmitting said recovery information to the base station by the handset.